1. Field of the Invention
The present invention relates in general to a method and apparatus for determining the concentration of components (such as water, oil, salt and sulfur) in a fluid mixture. More particularly, the present invention relates to a method and apparatus for determining the concentration of individual components within a mixture of fluids by utilizing the permittivity of the individual components when the permittivities of the different components are measurably distinct.
2. Description of the Related Art
Knowledge of accurate water content, salt content, sulfur content, and density of the hydrocarbon and chemicals for a fluid mixture is important for oilfield reservoir management, royalty allocation, buying and selling, corrosion management, refining, chemical processing, and aviation safety. An automated measuring device would be desirable for performing such determinations.
One possible means of measuring the ratio of fluids in a sample involves the use of radio frequency (RF) or microwave energy to determine the capacitance or permittivity of a fluid sample consisting of oil and water. Since these properties can be related to the ratio of water in oil, several devices based on this approach have been developed. However, several perturbing factors adversely influence currently available measurement means to yield highly erroneous data.
Water is an excellent solvent of salt and other contaminants found in petroleum based fluids. An unknown salt concentration typically gives rise to significant errors in the measurement of water concentration in a hydrocarbon fluid mixture. If the percentage of salt is known, then compensation for the salt content can be made. However, if the salt concentration varies over time, instrument error will be increased unless there is active compensation for the varying influence of the salt. Likewise, interactions between sulfur compounds and the water in a hydrocarbon fluid mixture will adversely influence the accuracy of the measurement of water concentration in the fluid mixture. Furthermore, the temperature and density variations of the fluid also influence the measurement of the water concentration. There is a present need for a simple, accurate, reliable, and stable means for determining the concentration of oil and other contaminants in a hydrocarbon fluid mixture, particularly given the major economic impact of incorrect measurements.
In most currently available microwave systems, a measurement is made of frequency changes in an oscillator circuit that are caused by impedance changes of the mixture as the concentrations of the components in the mixture change. Various components have different dielectric constants, which are proportionally related to the overall impedance of the mixture. Binary systems, such as water in oil, are relatively easy to measure. Increasing the number of components significantly, however, adds to the problem. Water is a solvent for many things, such as various salts that significantly affect the complex permittivity of the mixture.
The complex permittivity of many materials changes with the frequency used for the permittivity measurement. Thus, as the oscillator frequency is changed, the complex permittivity also changes and the resulting system of mathematical equations used to describe and solve for the component concentrations become increasingly non-linear. If, however, the permittivity measurements can be made at accurate and repeatable frequencies, the fluid system components could be determined from simple linear equations.
Newer microwave apparatus use multiple oscillators or voltage-controlled-oscillators (VCOs) to measure a wider range of water concentrations. As with any electronics, these oscillators are subject to drift due to the temperature of the ambient surroundings or from self-heating and aging of the components. It is difficult, or impossible, to separate drifts in the oscillator from actual impedance changes in the fluid medium; and, as explained previously, non-linear dielectric constants tend to magnify the measurement errors.
Some systems include a reference oscillator calibrated to provide a specific frequency for a known impedance, but the reference oscillator is subject to the same thermal and aging errors. In fact, component aging and thermal effects might have offsetting effects and move the reference frequency in the opposite direction from the measurement frequency. Thus, the reference and measurement oscillators require frequent calibration and recalibration.
Often measurements of hydrocarbon fluids (with admixed water and other contaminants) are made as the fluid is flowing from one location to another, such as into or out of a truck or ship, through a pipeline or from a wellhead. Thus, it is necessary to be able to measure the contaminants in the fluid as it is moving. As the fluid mixture moves through the measurement system, the relative concentrations of individual fluid components passing through the system will vary. Particularly in cases of laminar flow or irregular mixing, fluctuations in individual fluid components can change rapidly. Because the calculations of fluid concentrations assume stationarity (in a stochastic sense) or component fluid property constancy during the measurement time, it is important that the measurements be done such that an instantaneous “snapshot” of the fluid makeup is made. Ideally, several measurements would be made instantaneously, or at least fast enough so that the fluid's components could be considered constant during the period of measurement.
Various types of capacitance (radio frequency measurement), microwave (microwave energy measurement), and optical (spectrometer) apparatus have been used for measuring the concentration of one substance in another, particularly for measuring the water content in hydrocarbons.
Tassano in U.S. Pat. No. 4,112,744, Thompson in U.S. Pat. No. 4,266,188, and Scott et al. in U.S. Pat. No. 4,996,490 have described single frequency measuring devices. Tassano, in U.S. Pat. No. 4,112,744, discloses an apparatus for detecting water in oil that uses a capacitive probe immersed in a sample of pure oil similar to the oil in the fluid mixture under test. The apparatus alternatively connects a reference capacitor and the measuring probe capacitor to an electronic capacitance measuring circuit that produces an output signal indicating changes in the dielectric constant of the oil. In this system the reference capacitor will only compensate for the measuring circuit drift and aging, but not the pressure and temperature of the oil and water mixture. The single operating frequency of the unit is 50 kHz, rendering the unit incapable of dealing with variations in the measurement due to variations in salt and sulfur compounds.
Thompson in U.S. Pat. No. 4,266,188 discloses a method and apparatus for measuring a component, namely water, in a two-component flowing fluid mixture using a probe having three sets of sensor electrodes positioned in the mixture. One set of electrodes is placed into an elastic sack filled with water, the second set of electrodes is placed into an elastic sack filled with a pure oil similar to the type being measured, and the third set of electrodes are exposed to the fluid being measured. Each set of electrodes produces a signal representative of a measured electrical property of the liquid in which they are immersed, either resistivity or conductivity or alternatively capacitance or dielectric constant. Since any changes in temperature and pressure in the mixture being measured will affect the reading from all three sensors equally, the probe is considered self-adjusting so that the accuracy of the final measurement is relatively unaffected by these changes. Nevertheless, the measurement accuracy will be affected when there are changes in either oil composition or in water salinity of the actual mixture being measured. The operating frequency for the unit is not disclosed.
Scott et al. in U.S. Pat. No. 4,996,490 discloses an apparatus for measuring the concentration of one material, such as water, in another material, such as crude or refined oil, utilizing a microwave transmission line formed by a conduit for receiving the material and a center conductor sheathed with a dielectric covering. This covering operates to prevent short-circuiting the transmission path. An oscillator circuit is coupled to the transmission line and is driven by a free-running voltage controlled oscillator. A signal receiver monitors changes in frequency caused by impedance pulling of the oscillator due to the change in the dielectric constant of the mixture. Power transmitted to the fluid mixture and power reflected from the fluid are measured to determine whether an oil-in-water or water-in-oil emulsion is present and to verify the concentration of one fluid in the other for a particular single operating frequency. The operating frequency of this unit is not disclosed. The single frequency measurement of this device is unable to ascertain the effects of varying concentrations of salt or sulfur compounds in the fluid mixture.
Single frequency capacitance apparatuses have not been successful in measuring the water content of the hydrocarbon in high concentrations because salt, sulfur, density, and temperature adversely influence the capacitance reading.
One possible means of treating the problems described above for microwave or capacitance measuring devices involves the use of permittivity measurements at multiple frequencies, since such measurements permit inference of the salt or sulfur compound percentages in the fluid sample. This permits determination of the permittivity of the fluid mixture, which is a mathematically complex measurement in that it has both real and imaginary components, such as A+B×i, where A is the real component and B×i is the imaginary component, with i being the square root of −1.
Fluid complex permittivity measurements for monitoring of fluid concentration are influenced by multiple important components, including the measuring cell or sensor, the measuring electronics, and the physical model for complex permittivity of the fluid mixture.
The patents discussed below propose solutions to the problem of determining individual components of fluid mixtures. Each of these patents discloses different technical means for measurement, yet each patent is based on the same flawed concept (i.e., if the fluids constituting the mixture are exclusively crude oil and water, then, because the permittivities of crude oil and water are known and divergent with frequency change, the permittivity of the mixture can be measured and an algebraic formula used to find the ratio of the mixture components).
Helms et al. in U.S. Pat. No. 5,014,010 discloses a dual frequency microwave water content monitor. Microwave oscillators provide two different frequencies of microwave energy to an antenna, which transmits the microwave energy into a petroleum stream and receives microwave energy reflected back from the stream. The microwave energy provided by the antenna also passes through the petroleum stream and is received by another antenna. Both signal phase shift and attenuation are measured. Two frequencies are used to resolve ambiguities in signal phase shift. Measured signal attenuation and phase shift are used to determine the type of emulsion measured (i.e., oil-continuous or water-continuous). The preferred operating frequencies of the disclosed apparatus are 10.119 GHz and 10.369 GHz, although the two frequencies selected should be substantially different.
Cox in U.S. Pat. No. 5,033,289 and U.S. Pat. No. 5,272,444 discloses a water percentage monitoring means and method in which the water content of a petroleum-containing stream is measured by comparing a probe signal to a reference signal. A probe is located in the pipeline and is connected to the reference signal through a series resistance. The signal from the oscillator side of the resistor is converted into two reference signals: one with zero phase shift and one with 90 degrees phase shift. These two reference signals are mixed with the signal on the probe side of the resistor, which changes as a function of the complex electrical impedance, primarily capacitive, of the fluid stream. The real part of the complex impedance is measured with the zero phase shifted reference signal. This is the resistance/conductivity of the fluid, while the imaginary part of the complex impedance is obtained by mixing the 90 degree phase shifted reference with the probe side of the resistor to measure the capacitance/permittivity of the fluid. The resistance/conductivity versus the capacitance/permittivity measurements provide enough information to derive a water content value in a true oil-based emulsion or water-based emulsion without further contaminants such as salt. The operating frequency range of the Cox invention is 10–200 MHz, with the preferred operating frequency at approximately 20 MHz.
Agar in U.S. Pat. No. 5,101,163 discloses a device for measuring the concentration of two admixed fluid substances through the transmission of electromagnetic waves. The device utilizes a transmission element for transmitting a signal and two receiving elements for receiving the signal and providing first and second output signals. The system utilizes a receiving device for receiving the first and second output signals and measuring the ratio and/or the phase difference of the powers received by each receiver. Since oil absorbs very little energy compared to water, the amount of power received in each antenna is a function of the water content and the distance from the transmitting antenna.
Gaisford et al. in U.S. Pat. No. 5,103,181 discloses a composition monitor and monitoring process using impedance measurements with radio frequency bridge techniques to parameterize the complex dielectric properties of the fluids. The method uses the pipe with the mixture of fluids as a waveguide in which two transmission channels are established. These transmission channels are used as arms of a Wheatstone bridge that is balanced using variable phase shift and attenuation units. The operating frequency of the disclosed apparatus is in the range of 50 MHz–3 GHz.
Sinclair in U.S. Pat. No. 5,132,903 discloses a method and apparatus for analyzing oil and water mixtures in a well borehole, where the sensor is formed by two coupled lines. Because the dielectric constant of the tested fluid affects the coupling coefficient between the two lines, measuring transmitted signal power allows the fluid properties to be evaluated and converted to water content. The practical operating frequency range for the device ranges from 200 MHz–5 GHz, with a preferred operating frequency of about 2.5 GHz.
Agar et al. in U.S. Pat. No. 5,503,004 discloses a method and apparatus for measuring the percentages of oil and water present in a mixture. By measuring the energy absorption properties of the oil/water mixture, the percentages of oil and water present in the oil/water mixture can be determined regardless of whether the oil or the water is in the continuous phase and regardless of what the relative proportions of water and oil are. Measuring the energy absorption properties of the oil/water mixtures allows the apparatus to determine whether the oil or the water is in the continuous phase so that the proper data curve is selected and the percentage of water present can then be determined. The specified operating frequency of the disclosed apparatus is 2.45 GHz, but the possibility of using two or more distinct frequencies to obtain more information about the fluid's components is mentioned.
Arndt et al. in U.S. Pat. No. 5,596,150 and U.S. Pat. No. 5,675,259 disclose a method and apparatus for making complex permittivity measurements of mixed fluids including the use of a capacitive probe. The impedance of the probe is determined in part by the complex dielectric constant of the fluids between the probe electrodes. The percentage of fluid component present in the flow stream is identified from the permittivity variations of the flow stream. The operating frequency of the disclosed apparatus is approximately 1 GHz.
Scott et al. in U.S. Pat. No. 5,966,017 discloses devices, methods and systems using load-pulled electronic monitoring. The patent primarily discusses various probe configurations and probe terminations as used to measure various chemical substances. It discloses various techniques for chemical absorption/desorption as applied to microwave detectors and some variations on load-pull electronics. The transmission line based probe does not necessarily have to end in an open connection. Termination of this transmission line can be accomplished in several alternate ways: a resistor, capacitor, inductor, short, or diode. Several different operating frequencies were mentioned for the disclosed apparatus include 200 MHz, 400 MHz, 600 MHz, and 1.2–1.3 GHz.
Spectrographic optical apparatuses represent a new approach and have been used successfully in limited applications. However, optical apparatuses have not proven effective for midrange water concentrations. This type of device also requires extensive periodic recalibration, and measurement errors increase as the optical sources and/or sensors become dirty, as is often the case in oilfield applications.
None of the apparatuses disclosed or currently available are able to self-calibrate, automatically select and switch the frequency range, find the optimum frequency, and/or make rapid multiple measurements to accurately measure oil and water mixture ratios, salt and sulfur content, as well as the density of the fluid mixture. There is an existing need for a means for accurately determining the content of oil, water, salt and sulfur in complex fluid mixtures. There is a further need for a means determining individual components in fluid mixtures that is self-calibrating.